A fire truck drives past burning trees as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. Crews working to contain one of California's largest-ever wildfires gained some ground Monday against the flames threatening San Francisco's water supply, several towns near Yosemite National Park and historic giant sequoias. (AP Photo/Jae C. Hong)

Operations section chief Glen Stratton points to the Hetch Hetchy Reservoir on the map while briefing the media at the command post near Yosemite National Park, Calif., on Monday, Aug. 26, 2013. Crews working to contain one of California's largest-ever wildfires gained some ground Monday against the flames threatening San Francisco's water supply, several towns near Yosemite National Park and historic giant sequoias. (AP Photo/Jae C. Hong)

TUOLUMNE CITY, Calif. (AP) — Firefighters gained some ground Tuesday against the huge wildfire burning forest lands in the western Sierra Nevada, including parts of Yosemite National Park.

The 11-day-old blaze expanded to about 280 square miles, partly due to back burning by crews, but containment jumped to 20 percent.

The portion of the fire in Yosemite doubled to about 64 square miles but remained in backcountry, and the main attractions in the nearly 1,200-square-mile park remained open.

"The next couple of days are really going to be key for us," said California fire spokesman Daniel Berlant. "If the weather cooperates, and we see an increase in containment, we could really turn a corner on this stubborn fire."

The fire — now the seventh-largest California wildfire in records dating to 1932 — was threatening about 4,500 structures and has destroyed at least 23.

Thousands of firefighters have arrived since the fire erupted Aug. 17 west of Yosemite in the Stanislaus National Forest, where the slopes of the Sierra begin to rise above the eastern side of California's Central Valley.

An expected increase in humidity Tuesday afternoon could help suppress flames, said Matt Mehle, a National Weather Service meteorologist assigned to the fire.

Crews planned to focus Tuesday on the portion of the fire threatening communities in the north.

The fire approached the main reservoir serving San Francisco, but fears that the inferno could disrupt water or hydroelectric power to the city diminished.

Utility officials monitored the basin's clarity and used a new $4.6 billion gravity-operated pipeline system to move water quickly to reservoirs closer to the city.

So far the ash that has fallen onto the reservoir has not sunk as far as the intake valves, which are about halfway down the 300-foot O'Shaughnessy Dam. Utility officials said the ash is non-toxic but that the city will begin filtering water for customers if problems are detected.

Power generation there was shut down last week so firefighters would not be imperiled by live wires. San Francisco is buying replacement power from other sources to run City Hall and municipal buildings.

It has been at least 17 years since fire ravaged the northernmost stretch of Yosemite that now is under siege.

Crews cleared brush and set sprinklers on two groves of giant sequoias that were less than 10 miles away from the fire's front lines, said park spokesman Scott Gediman. While sequoias have a chemical in their bark to help them resist fire, they can be damaged when flames move through slowly with such intense heat.

The fire has swept through steep Sierra Nevada river canyons and stands of thick oak and pine, closing in on Tuolumne City and other mountain communities. It has confounded ground crews with its 300-foot walls of flame and the way it has jumped from treetop to treetop.

Meanwhile, biologists with the Forest Service are studying the effect on wildlife. Much of the area that has burned is part of the state's winter-range deer habitat. Biologist Crispin Holland said most of the large deer herds would still be well above the fire danger.

Biologists discovered stranded Western pond turtles on national forest land near the edge of Yosemite. Their marshy meadow had burned, and the surviving creatures were huddled in the middle of the expanse in what little water remained.

"We're hoping to deliver some water to those turtles," Holland said. "We might also drag some brush in to give them cover."

Wildlife officials were also trying to monitor at least four bald eagle nests in the fire-stricken area.

Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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